This invention relates to apparatus useful in sensing the depth of cellular pits or cells formed in a surface. The apparatus is particularly useful in the preparation of rotogravure cylinders.
In the production of rotogravure cylinders to be used in an intaglio printing process, a plurality of closely adjacently spaced cells or cup-shaped pits are formed in the cylinder surface, either by electromechanical means or by means of an etching process. During the printing operation, the cells formed receive the printing ink and establish varying intensities of color shade depending upon their volume, so that a desired pattern including respective variations of color tone intensity can be produced. Thus, the intensity of the print depends upon the quantity of printing ink contained in the individual cells, i.e. on the section area and/or the depth of the cells. Therefore, it is of principle importance in the production of the rotogravure cylinders that the cells be formed to have a desired depth and thus provide a faithful color reproduction of the image of an original.
It has heretofore been customary to sense and inspect engraved cells or etched cells with respect to their depth or their volume by means of a microscope. Furthermore, it is possible to scan and record the profile of a predetermined number of cells by using profile recorders and with the aid of a special stylus. The values obtained in this way can then be compared with an empirically established scale of values in order to determine whether the examined cells have the necessary depth. However, these measuring methods are extremely cumbersome and time-consuming.
There is disclosed and claimed in copending application Ser. No. 371,467, filed June 19, 1973, and application Ser. No. 219,991, filed in 1972, an improved method for determining the depth of cells formed in a material surface. In accordance with the method, cells of an individual region are scanned successively by a bundle of an ultrasonic beam which is projected onto the surface of the material layer in a plane perpendicular to the side walls or side wall portions of the cells and at a specific angle with respect to the surface of the material layer. A portion of the ultrasonic beam is reflected back towards a source by the side wall and bottom portions of the scanned cell regions so that the reflected portion of the ultrasonic beam is parallel to the incident beam. As explained in the above-identified copending patent applications, the intensity or amount of the reflected ultrasonic beam depends upon the size of the area of the wall of the cell and hence on the depth of the cell because with a given diameter of ultrasonic beam and with increasing wall size or depth of the cell, an increased portion of the incident ultrasonic beam is returned to the receiver and measured thereby.